Some items of large mining machinery, such as mining shovels and electric or hydraulic excavators, are fitted with crawler tracks for locomotion. The individual links in the crawler tracks are known as “shoes”. These track shoes, also known as “crawler shoes”, are linked together by pivot pins to form an endless articulated track, each track being mounted on a row of support rollers. Drive sprockets at either end of the row of support rollers engage lugs on the crawler shoes to drive the track. The rollers roll upon the ground-engaging track shoes of the track as the track is driven by the sprockets. Each roller may be provided with dual rims.
The weight of the machinery is borne by the particular track shoe(s) upon which the rollers bear. As the rollers move from one track shoe to another, the weight of the machinery is transferred between shoes. In view of the large weight being carried, any point loads or concentrated loads must be avoided, as such loads may crack or distort a track shoe, or otherwise cause excessive wear.
There are various known crawler shoe designs. FIG. 1 illustrates crawler shoes of a first known type. As can be seen in that drawing, each shoe has a pair of roller paths “A” (shown cross-hatched) along which dual rollers travel, and a single drive lug “B” which is engaged by a sprocket in use. One disadvantage of dual roller path/single drive lug shoes is that the drive lug often wears out before the roller paths.
The known track shoes shown in FIG. 1 have a longitudinal split “C” in each roller path. The aim of the longitudinal split is to transfer approximately half of the load from one shoe to the next. In theory, the load is split evenly between two shoes on either side of the longitudinal split. As the roller moves from one shoe to the next, half the load is initially transferred across the longitudinal split to the next roller, followed shortly thereafter by the other half of the load. The load transfers in the two roller paths occur simultaneously.
However, it has been found in practice that the longitudinal split in each roller path causes accelerated wear and metal flow in the crawler shoes. As the machinery may be operating on sloping or undulating ground, or over rocks, and since the wear characteristics of adjacent crawler shoes are not identical, the high contact pressure applied by a roller on a roller path is not distributed evenly across the longitudinal split. The effect of concentrating the weight on a shoe on one side only of the longitudinal split, and particularly on the edge portion of the shoe near the longitudinal split, is to cause premature metal flow or “toe nailing” of the track shoes along the roller path. This in turn causes the track shoes to bind with mating shoes, which can result in cracking and failure of the track shoe.
Another type of crawler shoe design can be found in international patent application no. PCT/AU99/00156. The track shoe illustrated in that patent application has a single roller path and dual drive lugs located on opposite sides of the roller path. A disadvantage of dual drive lug/single roller path designs is that the roller path often wears out before the drive lugs. Further, the maximum propel speed of the machine is limited due to the rough transition of load rollers between shoes.
It is an object of this invention to provide an improved crawler shoe which overcomes or ameliorates the above described disadvantages, or which at least provides the consumer with a useful choice.